Regional hydrological processes have been greatly influenced by human activities and climate variability. The inflow of Panjiakou Reservoir, which is the largest reservoir located on the Luan River, has shown a significant decreasing trend over the past 50 years. A large-scale hydrological model, namely the geomorphology-based hydrological model (GBHM), and a climate elasticity model were applied to perform quantitative attributing analysis of runoff change in the study catchment. Annual runoff decreased by 19.5 mm from 65.7 mm in the period 1956–1979 to 46.2 mm in the period 1980–2005. Annual direct water intake increased by 22.5 mm from 3.6 to 26.1 mm. Climate impact was accountable for the runoff increase of 8.8 and 9.2 mm simulated by GBHM and the climate elasticity model, respectively. Impacts of land use and vegetation change accounted for the runoff decrease of 2.5 mm. Change of precipitation and vegetation cover contributed to annual runoff change for the upper catchment (grassland-dominated). Change of antecedent precipitation (a proxy of soil moisture) also contributed to annual runoff change for the lower catchment (forest-dominated) and the whole catchment (mixture vegetation).